The present invention relates to direct conversion receivers and associated bandpass processing assemblies and, more particularly, relates to direct conversion receivers and associated bandpass processing assemblies capable of compensating offsets in the direct conversion receivers and associated bandpass processing assemblies.
The general trend in portable radio communication apparatus is the reduction in volume, weight and power consumption of such devices. This has led to efforts toward reducing the number of elements necessary to perform the functions associated with portable communications devices. In particular, the radio frequency front end of portable devices, which typically comprises a number of down-converting stages, is an area in which a reduction in the number of elements would be beneficial.
One approach to reduce the number of stages in the radio frequency front end is to convert a received radio frequency carrier signal down to a DC intermediate frequency (zero IF) in a single step. This is termed direct conversion and is carried out in receivers known by any one of the terms homodyne or zero IF receivers, as well as direct conversion receivers. In a direct conversion receiver, received radio frequency signals are converted directly into base band signals such that separate intermediate frequency stages are not required. Therefore, the number of higher frequency components needed in a direct conversion receiver is less than in conventional receivers, which include intermediate frequency stages. Due to less complexity, the degree of integration of direct conversion receivers can be increased compared to receivers that must include intermediate frequency stages.
To carry out direct conversion, a local oscillator signal (LO) having the same frequency as the radio frequency carrier signal (i.e., the LO is “on-channel”) is mixed with the carrier signal in a suitable non-linear device such as a mixer diode. The output of the mixer contains the sum and difference of the LO and the carrier signal. Thus, a mixer product exists at twice the carrier signal, and also at DC (zero Hz). The high frequency mixer products can be filtered by a suitable low pass filter. Once the radio frequency carrier signal has been down-converted, the modulating signal may be de-modulated using an appropriate demodulator, e.g., an I/Q demodulator for an I/Q modulating signal, or an FM demodulator for an FM signal.
In the field of radio telephony, particularly cellular telephony, use of a direct conversion receiver is not without certain drawbacks. One of the main problems of using a direct conversion receiver in a cellular radiotelephone, and a problem that is widely recognized, is that of DC voltage offset. DC offset basically consists of unwanted DC being provided at the output of the RF front-end stage which, if large enough, causes distortion to the wanted signal. Because DC is encompassed in the IF bandwidth, the DC offset present at the RF front-end output and that contributed by IF amplifiers severely limits the sensitivity of the receiver if it is not removed. Low frequency AC, such as flicker noise and spurious AM demodulation, can also cause similar problems.
The dynamic range of the receiver is adversely affected by the fact that, in addition to the high frequency signal of the reception channel, the mixer of the receiver also receives high frequency signals of channels adjacent to the reception channel. And due to the non-ideality of the mixers, a disturbing DC offset is produced at the output of the mixer. As the strength of the signal of the adjacent channels increases, then, the stronger signals are mixed with the signals of the reception channel. Thus, a stronger signal of the adjacent channels can produce substantially higher DC offset in the signal than the desired signal expressed on the reception channel.
Generally, DC offsets can be divided into two groups: (1) dynamic offsets, and (2) static offsets. Dynamic offsets are typically a function of radio frequency and local oscillator signals in the receiver, as well as undesirable single-tones and gain switching transitions in the receiver. In this regard, dynamic offsets generally change over time. Static offsets, which are independent of the signals and gain switching transitions, can be a function of process variations, such as temperature, supply voltage and DC operation conditions of the elements of the receiver.
A number of different methods to solve the problem caused by DC offset have been investigated. The most common methods include: AC coupling, closed-loop servo correction, and DC averaging and removal.
Of the most common methods, AC coupling is the simplest approach. The IF stages are AC coupled to remove the DC voltage and low frequency noise and stop it from propagating up to the highest gain stages. AC coupling, however, introduces a notch into the center of the IF pass band. In the case of modulation such as FM, the carrier term is removed so distortion is introduced into the required signal. Such distortion is a significant problem with direct conversion: the interference cannot easily be differentiated from the wanted signal. Often a major problem with AC coupling is that the coupling capacitors can take a significant time to charge up which means that the receiver can take tens of milliseconds to settle. In this regard, pre-charging techniques are often required. When a narrow band filter is used, the settling time becomes long because the filter cannot react to quick changes in power. On the other hand, with a wide band filter, it is possible to achieve a short settling time, but a filter of this kind also filters a substantial part of the useful signal, thereby reducing the performance of the receiver.
Of the other most common methods to solve the problem caused by DC offset, closed-loop servo correction has been used in audio amplifiers to remove offset voltages. It can also be used in a direct conversion receiver to remove the DC offset created in cascaded IF stages and the mixers. Careful design, however, is required to ensure stability. Another common method, DC averaging and removal, is usually performed by a digital signal processor (DSP). According to such a solution, the DC count component of the signal is averaged over a relatively long time frame. The average is then subtracted from the wanted signal. DC averaging and removal is broadly equivalent to AC coupling and, as such, can potentially introduce distortion. It does have the advantage over AC coupling, however, in that relatively long average and time (and, hence, very low cut-off frequencies) can be achieved without the need for high impedance design and/or large value coupling capacitors.
Taking the example of an IQ modulated signal, two consecutive symbols are combined into I and Q signals. Thus, I and Q signals are produced from the received signal in the IQ demodulator. And from the produced I and Q signals, a decision is made in the receiver as to which symbol pair (00, 01, 10, 11) has been transmitted. The decision as to whether the transmitted symbol is a 0 or a 1 is made on the basis of the voltage level of the demodulated signal. DC offset can occur in both I and Q signals, which can lead to a wrong decision in the receiver as to the signal pair transmitted. In an extreme case, even the error correction logic of the receiver cannot correct the information that has a faulty expression. In some prior solutions, an attempt is made to express the signal of the reception channel in spite of high interfering DC offset. The drawback of these solutions, however, is the fact that they only operate in situations where the disturbing DC offset is constant or changes very slowly. In situations where the power of the signals in the adjacent channels vary quickly, the disturbing DC offset also changes quickly. As such, the prior solutions are not capable of fully eliminating the disturbance caused by the DC offset. So while the prior solutions are effective in mitigating DC offset in certain circumstances, such solutions do not satisfactorily tackle the problem across a wide range of differing received signal conditions.